The present invention relates to power control systems and, more specifically, to a method and apparatus for controlling the supply of three-phase power to a three-phase motor.
Three-phase power motors are frequently used in applications that require increased power and efficiency. In a three-phase power system, three phases of power are delivered to the motor along three separate wires. Because the sine waves for each phase are 120 degrees apart, when the current or voltage of one phase is at zero the other two phases are still supplying power. Further, since the power supply to the motor is the sum of the power of the three phases, the power to the motor is constant at all times.
One of the primary reasons for using a three-phase power system is to reduce energy consumption. A three-phase motor requires significantly less power to operate than a comparable single-phase motor. These power savings can be reduced, however, by variations in the voltage supply to the motor. Voltage imbalances between the three phases are common, and largely due to variations on the part of the utility lines supplying the power.
A significant problem with the use of three-phase motors is that they are designed to work most efficiently at rated loads. When varying loads are applied to the motor, a significant amount of power is wasted by the system. Specifically, the motor continues to draw current based on the applied voltage to the motor even when the motor is under no load. As a result, it is desirable to have a control system that decreases the voltage to the motor when the motor is unloaded or only partially loaded.
Power control systems are widely known in the electronics field. These systems generally use measurements of current and voltage to control the motor. The current and voltage of a particular motor are used to calculate a power factor. The value of the calculated power factor is then compared with a target power factor value stored in the system. If the two do not agree, the voltage of the motor is adjusted and the power factor is recalculated. This cycle continues until the two values are the same.
Methods have been developed in the past for controlling the power in three-phase power systems. Generally, these methods require current to be measured separately for each phase. Since current is sinusoidal in three-phase motors, there will be points when the current for each phase is zero. These prior art control methods base operational decisions on the points when current is equal to zero, or the zero-crossing points. Since each cycle will have two such points, the prior art methods generally have six points per cycle where delays are calculated.
These prior art systems have at least one serious limitation, however. In a three-phase system, any variation in one phase will alter the current zero-crossing points in the other two phases. Variations in the incoming power system typically found in an industrial environment in the form of electrical noise, droops, and surges on a single phase will alter delays calculated for power factor correction. These individual phase variations consequently result in instability in the three-phase system. For example, a delay of phase one is calculated based on its zero-crossing point. This power factor correction delay for phase one when applied to the zero crossing of phase two, which has been perturbed by an electrical disturbance will effect the next calculation for phase one. When this incorrect power factor correction calculation is applied the systems tend to oscillate. Therefore, instability is introduced into the system. To prevent this instability, these types of prior art systems respond slowly to load changes, which can significantly reduce the applicability of these systems with motors that have sudden load changes.
It is therefore desirable to have a three-phase power control system with limited instability that can be used in motors that experience sudden load changes. It is further desirable to have a three-phase power control system which is responsive to voltage imbalances and which adjusts for such imbalances.
The aforementioned problems are overcome by the present invention which provides a method for controlling the supply of three-phase power to a three-phase motor by calculating phase trigger firing delays for all six zero-crossing points based on the measurement of one zero-crossing point of one phase. The method includes the general steps of (a) measuring the zero-crossing point of a single phase, (b) calculating the timing of the phase triggers for all three phases based on this single phase measurement, and (d) firing the phase triggers for all three phases based on the calculated sequence timing. To provide power factor correction, the present invention may further include the steps of (a) measuring power factor to determine whether it is within predetermined limits and (b) if the power factor is not within predetermined limits, introducing a delay into the timing of the phase triggers for all three phases.
In one embodiment, the method further includes steps for balancing the voltage of the three separate phases. In this embodiment, the method further includes the steps of (a) measuring the voltage for each phase on the supply side, (b) comparing the measured values and (c) adjusting the timing of each phase trigger to account for voltage imbalances in the supply lines.
The present invention further provides a power control system for controlling the supply of three-phase power. In one embodiment, the system generally includes a series of phase triggers for controlling the supply of three-phase power, a zero-crossing detector for detecting a zero-crossing point of the voltage of a single phase, a microprocessor for calculating the timing of the firing sequence for all of the phase triggers based on the timing of the detected zero-crossing of a single phase and for firing the phase triggers in accordance with the calculated firing sequence.
In one embodiment, the system also includes a zero-crossing detector for detecting the zero-crossing point of the current. In this embodiment, the microprocessor is programmed to determine the power factor based on the measured timing of the current and voltage zero-crossings. The microprocessor is further programmed to reduce the voltage applied to the motor by introducing a delay into the timing of the phase triggers if the power factor does not satisfy predetermined criteria.
In a further embodiment, the system includes phase balancing components that function to provide a degree of balance to the three phases presented to the motor. In this embodiment, the system includes a converter on each phase for converting the current into a DC voltage. The system further includes a plurality of comparators for comparing the voltages of the three phases. The microprocessor is programmed to monitor the output of the comparators and, if the imbalance falls outside of predetermined limits, introduce delay (or additional delay) into the timing of the firing of the phase triggers to move all three phases toward balance.
In one embodiment, the phase balancing components include a deadband operator that permits the phase balancing components to introduce a balancing delay only when predetermined characteristics have been satisfied. In one embodiment, the deadband operator is embodied within the microprocessor. In this embodiment, the microprocessor is programmed to track the number of cycles during which the voltage of one phase exceeds another, and to introduce a delay only when this number exceeds a predetermined value. Accordingly, the phase balancing components can purposeful ignore momentary imbalances caused by transient events.
The present invention provides a simple and effective method and apparatus for providing control of three-phase power with increased stability and efficiency. The increased stability allows the invention to be used in systems that experience sudden load changes. The increased efficiency decreases the overall power consumption of the motor.
These and other objects, advantages, and features of the invention will be readily understood and appreciated by reference to the detailed description of the preferred embodiment and the drawings.